UV-blocking borosilicate glass, the use of the same, and a fluorescent lamp

ABSTRACT

The invention relates to a borosilicate glass having the following composition (in wt. % based on oxide content): between 55 and 80 of SiO 2 ; between 8 and 25 of B 2 O 3 ; between 0.5 and 10 of Al 2 O 3 ; between 1 and 16 of Li 2 O+Na 2 O+K 2 O; between 0 and 6 of MgO+CaO+SrO+BaO; between 0 and 3 of ZnO; between 0 and 5 of ZrO 2 ; between 0 and 5 of Bi 2 O 3 ; and between 0 and 3 of MoO 3 ; the sum of the Bi 2 O 3  and MoO 3  amounting to between 0.01 and 5. The invention also relates to a fluorescent lamp, especially a miniature fluorescent lamp.

The invention relates to a UV-blocking borosilicate glass and to the useof the same. The invention also relates to a fluorescent lamp.

Fluorescent lamps, known as backlights, are used as backgroundillumination for, for example, displays, for example of personalcomputers, laptops, palmtops, vehicle navigation systems.

Typical sizes of miniaturized lamps of this type are an externaldiameter of between 2 and 5 mm. Typical internal diameters are between1.8 and 4.8 mm.

Whereas standard fluorescent tubes consist of a soft glass which has avery low solarization stability, glasses which are moresolarization-stable are used for backlights, the basic structure ofwhich corresponds to that of fluorescent tubes, in order to ensurelong-term functionality.

On account of the structure of the backlights, the glasses used have tobe suitable for vacuum-tight fusing to a metal or metal alloy used inlamp manufacture. For this purpose, they have to have a thermalexpansion which is matched to the thermal expansion of the metal ormetal alloy. For example, if tungsten is used, given the coefficient ofthermal expansion α_(20/300) of W of 4.4×10⁻⁶/K, glasses with α_(20/300)of between 3.4×10⁻⁶/K and 4.3×10⁻⁶/K are particularly suitable. By wayof example, if Kovar, an Fe—Co—Ni alloy, is used, glasses withα_(20/300) of between 4.3×10⁻⁶/K and 6.0×10⁻⁶/K are eminently suitable.

The glasses should Lend to have low working points V_(A), i.e.V_(A)<1200° C., to allow them to be worked at relatively lowtemperatures. The transformation temperature T_(g) should be matched tothe melting characteristics of the metal or metal alloy to which it isto be fused. For example, in the case of fusing to Kovar, thetransformation temperature should preferably be between 440° C. and 530°C. T_(g) of up to 580° C. is eminently suitable for fusing to tungsten.

The transmission profile is a significant property of glasses. In thevisible region, the highest possible light transmission is required, inorder to obtain a high light yield from the lamp, whereas in the UVregion no transmission or only a low transmission is the aim; in orderfor the minimum possible amount of the harmful UV radiation to beallowed to pass through. The UV-blocking requirements depend on the usesof the glasses. For example, if they are used as lamp glasses forfluorescent lamps, in particular the Hg line at 253 nm should beblocked.

For example, for backlights, a high UV blocking≦253 nm is desirable inorder to ensure that irradiated plastic parts, for example in laptops,do not become yellow and embrittled. This requirement is met by glasseshaving a UV transmission at λ≦254 nm of τ≦0.1%, measured on specimenswhich are 0.2 mm thick. For other uses, an UV transmission τ≦0.1% atλ≦240 nm is sufficient. In any event, the transition from the wavelengthrange which is not transmitted to the wavelength range which istransmitted should be as short as possible, i.e. the transmission curveshould be as steep as possible in this region.

The minimum demand imposed on the transmission in the visible wavelengthregion is, at λ>400 nm and with a specimen thickness of 0.2 mm, atransmission of 90%. Therefore, the requirement is τ (>400 nm; 0.2mm)≧90%.

A further important property of glasses for fluorescent lamps, inparticular for backlights, is the resistance to solarization which isrequired in order to allow a long lamp service life to be achieved, i.e.a light yield which remains as constant as possible. In the presentcontext, the term “solarization-stable” is to be understood asencompassing glasses which have a drop in transmission of at most 5% at300 nm after 15 hours' HOK-4 irradiation, i.e. irradiation with an Hghigh-pressure lamp with a main emission of 365 nm and an irradiationstrength of 850 μW/cm² at 200 to 280 nm at a distance of 1 m on a glassspecimen which is 0.2 mm thick.

The patent literature has already disclosed various documents whichdescribe more or less UV-blocked glasses, in particular lamp glasses.However, these glasses have certain drawbacks, in particular aUV-blocking action which does not comply with the high demands imposednowadays.

The borosilicate glass for discharge lamps which is described in JP8-12369 A contains, for UV blocking purposes, a total of from 0.03 to 3%by weight of at least two of the four components V₂O₅, Fe₂O₃, TiO₂ andCeO₂. A high transmission and a high solarization resistance cannot beset using these components, in some cases in high individualproportions, and combinations thereof.

U.S. Pat. No. 5,747,399 describes a glass for miniaturized fluorescentlamps which is supposed to retain its solarization stability and itsUV-impermeability by means of TiO₂ and/or PbO and/or Sb₂O₃. However,doping with TiO₂, in particular high levels of the latter, leads todiscoloration of the class. PbO should also not be used, on account ofthe associated environmental problems.

Therefore, it is an object of the present invention to provide a glasshaving a high transmission in the visible region (>400 nm) and a highlevel of blocking in the UV region (≦240 nm), and also having a thermalexpansion which is matched to the expansion of tungsten or Kovar.

The object is achieved by a borosilicate glass in accordance with themain claim.

A glass having the desired transmission properties comprises the baseglass system (in % by weight, based on oxide): 55 to 80 SiO₂, 8 to 25B₂O₃, 0.5 to 10 Al₂O₃, 1 to 16 Li₂O+Na₂O+K₂O, 0 to 6 MgO+CaO+SrO+BaO, 0to 3 ZnO and 0 to 5 ZrO₂.

The presence of MoO₃ and/or Bi₂O₃, specifically in a total amount offrom 0.01 to 5% by weight, with from 0 to 3% of MoO₃ and from 0 to 5% ofBi₂O₃, is crucial to the invention.

The minimum level of MoO₃ and/or Bi₂O₃ is a requirement in order toachieve the high UV blocking. Higher levels of MoO₃ and/or Bi₂O₃ wouldlead to discoloration of the glass. A minimum total amount of 0.1% byweight is preferred, and a minimum total amount of 0.2% by weight isparticularly preferred, as is a maximum total amount of 3% by weight. Aminimum MoO₃ content of 0.4% by weight or a minimum Bi₂O₃ content of1.0% by weight is particularly preferred. Bi₂O₃ also greatly improvesthe solarization stability of the glass. In particular in theparticularly preferred embodiments, it is possible to achieve UVblocking up to 254 nm, i.e. a τ≦0.1% at τ≦254 nm with a specimenthickness of 0.2 mm. A minimum MoO₂ content of 0.6% by weight or aminimum Bi₂O₃ content of 1.3% by weight is very particularly preferred.

The glass preferably comprises the glass system (in % by weight, basedon oxide): SiO₂ 55-79, B₂O₃ 10-25, Al₂O₃ 0.5-10, Li₂O+Na₂O+K₂O 1-16,MgO+CaO+SrO+BaO 0-6, ZnO 0-3, ZrO₂ 0-1; Bi₂O₃ 0-5, MoO₃ 0-3; withBiO₂+MoO₃ 0.1-5.

It is particularly preferred for the glass to comprise the followingglass system: SiO₂ 55-79, B₂O₃ 8-12.5; Al₂O3 0.5-10; Li₂O+Na₂O+K₂O 1-16;MgO+CaO+SrO+BaO 0-6; ZnO 0-3; ZrO₂ 0-3; Bi₂O₃ 0-5; MoO₃ 0-3; withBi₂O₃+MoO₃ 0.01-5.

It is preferable not to add ZrO₂, so that the glass is ZrO₂-free, apartfrom inevitable impurities resulting from raw materials or tank furnacecorrosion.

Glasses from the abovementioned composition ranges containing from70-80% by weight of SiO₂ have coefficients of thermal expansionα_(20/300) of between 3.4×10⁻⁶/K and 4.3×10⁻⁶/K and are thereforeparticularly suitable for fusing to tungsten.

Glasses from the composition range (in % by weight, based on oxide) SiO₂73-79, B₂O₃ 12.5-25; Al₂O₃ 0.5-10; Li₂O+Na₂C+K₂O 1-11; MgO+CaO+SrO+BaO0-6; ZnO 0-3; ZrO₂ 0-5; Bi₂O₃ 0-5; MoO₃ 0-3; with Bi₂O₃+MoO₃ 0.01-5 areparticularly preferred for fusing to tungsten.

Glasses from the abovementioned composition ranges containing from55-75% by weight of SiO₂ have coefficients of thermal expansion ofbetween 4.3×10⁻⁶/K and 6.0×10⁻⁶/K and are therefore particularlysuitable for fusing to Kovar.

Glasses from the composition range (in % by weight, based on oxide) SiO₂55-73; B₂O₃ 15-25; Al₂O₃ 1-10; Li₂O+Na₂O+K₂O 4-16; MgO+CaO+SrO+BaO 0-6;ZnO 0-3; ZrO₂ 0-5; Bi₂O₃ 0-5; MoO₃ 0-3; with Bi₂O₃+MoO₃ 0.01-5 areparticularly preferred for fusing to Kovar.

The glass may contain the usual quantities of standard refining agents,such as for example evaporation refining agents, such as Cl⁻ and F⁻, butalso Redox refining agents, which are active on account of theirpolyvalent cations, e.g. SnO₂ and Sb₂O₃. It is preferable for the glassto contain 0-1% by weight of Sb₂O₃, 0-1% by weight of As₂O₃, 0-1% byweight of SnO₂, 0-1% by weight of CeO₂, 0-0.5% by weight of Cl, 0-1% byweight of F, 0-0.5 of sulfate, given as SO₃.

CeO₂ assists with the refining but may have an adverse effect on thesolarization stability if it is present in excessive quantity.

Furthermore, the glass may contain 0-5% by weight of TiO₂, preferably0-1% by weight of TiO₂, and 0-3% by weight of PbO. TiO₂ assists MoO₃ andBi₂ O₃ by shifting the UV edge, i.e. the transition between absorptionand transmission, into the longer-wave range. This makes it possible toachieve UV-blocking actions not only up to 240 nm, but even up to 254 nmand above, even with only the abovementioned low levels of MoO₃ and/orBi₂O₃. The doping according to the invention, compared to the TiO₂doping of the prior art, makes it possible to dispense altogether withTiO₂ or to add it in such small quantities that its disruptivediscoloration plays no role.

The glass may contain up to 1% by weight of Fe₂O₃ without this havingany disadvantageous effect. Fe₂O₃ also contributes to shifting theabsorption edge into the longer-wave region.

The glass may also contain small proportions, which have no adverseeffect on the glass system, of V₂O₅, Nb₂O₅ and WO₃.

The total quantity of Fe₂O₃, CeO₂, V₂O₅, Nb₂O₅, WO₃, TiO₂, PbO, As₂O₃,Sb₂O₃ should not exceed 5% by weight, since otherwise the glass isexcessively discolored in the visible region.

EXEMPLARY EMBODIMENTS

Standard raw materials were used to produce the example glasses and thecomparison glasses.

The well-homogenized batch was melted, refined and homogenized in thelaboratory in a quartz glass crucible at 1600° C. Then, the glass wascast and cooled at 20 K/h.

The table shows thirteen examples of glasses according to the invention(A1 to A13) and two comparative examples (C1, C2) including theircompositions (in % by weight, based on oxide) and their main properties.

The following properties are given in the table:

-   -   the coefficient of thermal expansion α_(20/300) [10⁻⁶/K]    -   the transformation temperature T_(g) [° C.]    -   the working point V_(A) [° C.]    -   the softening point E_(W) [° C.]    -   the highest wavelength at which τ is at most 0.1% (for a        specimen thickness of 0.2 mm) to document the blocking in the UV        region (“UV blocking”)

TABLE Compositions (in % by weight, based on oxide) and importantproperties of glasses according to the invention (A) and of comparisonglasses (C) C1 C2 A1 A2 A3 A4 A5 A6 A7 SiO₂ 68.45 67.65 72.6 78.0 72.068.0 59.0 69.0 68.25 B₂O₃ 19.0 19.0 14.5 10.8 16.0 12.0 16.0 8.0 19.0Al₂O₃ 2.65 2.65 2.0 2.0 3.0 1.0 2.0 5.0 2.65 Na₂O 0.8 0.8 2.0 2.0 2.01.0 1.0 1.0 0.8 K₂O 7.7 7.7 — 1.0 1.0 3.0 3.0 3.0 7.7 MgO — — 0.5 1.01.0 4.0 4.0 4.0 — CaO — — 2.0 0.9 1.9 1.9 3.9 1.9 — SrO — — 1.6 1.0 1.01.0 1.0 1.0 — BaO — — — — — 5.0 5.0 2.0 — Li₂O 0.7 0.7 1.0 1.0 1.0 2.02.0 2.0 0.7 ZnO 0.6 0.6 2.4 1.0 1.0 1.05 2.0 2.0 0.6 As₂O₃ 0.1 0.1 0.1 —0.1 — 0.1 0.1 0.1 Sb₂O₃ — — — 0.10 — — — — — Cl — — — — — 0.05 — — —TiO₂ — 0.8 — — — — 0.8 — — Bi₂O₃ — — 1.30 — 0.80 — 0.2 0.7 0.2 MoO₃ — —— 1.20 — 0.60 — 0.3 — α_(20/300) [10⁻⁶/K] 4.68 4.73 3.45 3.42 3.77 5.245.78 4.97 4.7 Tg [° C.] 485 491 505 515 502 455 449 487 485 V_(A) [° C.]1055 1053 1078 1190 1062 872 729 999 1050 E_(W) [° C.] 720 715 761 791751 626 588 690 720 UV-blocking [nm] <240 261 254 268 250 254 265 251242 A8 A9 A10 A11 A12 A13 SiO₂ 67.95 66.45 67.65 67.95 67.45 67.85 B₂O₃19.0 19.0 19.0 19.0 19.0 19.0 Al₂O₃ 2.65 2.65 2.65 2.65 2.65 2.65 Na₂O0.8 0.8 0.8 0.8 0.8 0.8 K₂O 7.7 7.7 7.7 7.7 7.7 7.7 MgO — — — — — — CaO— — — — — — SrO — — — — — — BaO — — — — — — Li₂O 0.7 0.7 0.7 0.7 0.650.65 ZnO 0.6 0.6 0.6 0.6 0.60 0.60 As₂O₃ 0.1 0.1 0.1 0.1 0.10 0.10 Sb₂O₃— — — — — — Cl — — — — — — TiO₂ — — — — — — Bi₂O₃ — 2.0 0.8 — — 0.2 MoO₃0.5 — — 0.8 1.0 0.4 α_(20/300) [10⁻⁶/K] 4.72 4.81 4.8 4.8 4.86 4.73 Tg[° C.] 485 480 487 485 468 485 V_(A) [° C.] 1050 1040 1053 1050 10511050 E_(W) [° C.] 720 720 720 720 710 720 UV-blocking [nm] 248 259 248255 262 251

Comparative Example C1 has a UV edge at too low a wavelength, i.e. itdoes not sufficiently block the UV region.

The TiO₂-containing Comparative Example C2 has a good UV-blockingaction, as is also achieved by the doped glasses without the addition ofTiO₂ in accordance with the invention.

Exemplary embodiments A1, A3, A7, A9 and A20 show glasses doped purelywith Bi₂O₃. A2, A4, A8, A11 and A12 show glasses doped purely with MoO₃.A6 and A13 are examples of mixed doping with Bi₂O₃ and MoO₃. A5 revealsthe boosting action of the optional component TiO₂ or, compared to C2,the improvement in the blocking achieved by the invention without itbeing necessary to increase the TiO₂ content.

The glasses according to the invention have a high resistance tosolarization, expressed by Δ₁₅τ (300 nm; 0.2 mm) of <5%, a hightransmission in the visible region, expressed by τ (>400 nm; 0.2 mm)≧90%and a good UV-blocking action, in particular expressed by τ (≦240 nm;0.2 mm)≦0.1% or by the detail giving the highest wavelength at which τis at most 0.1% (specimen thickness 0.2 mm). This wavelength is 240 nmor more.

In the preferred embodiments, the glasses have a UV transmission atλ≦254 nm of τ≦0.1%.

Furthermore, the glasses have a working point V_(A) of <1200° C., sothat they can be worked successfully.

The glasses have transformation temperatures T_(g) of between 440° C.and 580° C. They are therefore suitable for fusing to Kovar, for whichpurpose it is preferable to use the glasses with T_(g) of between 440°C. and 530° C., or to, tungsten, for which purpose it is preferable touse the glasses with a higher T_(g).

Furthermore, the glasses have a coefficient of thermal expansionα_(20/300) of between 3.4×10⁻⁶/K and 6.0×10⁻⁶/K. They are thereforesufficiently well matched to the thermal expansion of tungsten or Kovar,i.e. can be fused to one of these materials in a vacuum-tight manner.

With these properties and with τ≦0.1% at λ≦254 nm, the glasses areeminently suitable for the production of fluorescent lamps.

The glasses have a high resistance to crystallization. Consequently, theglasses are eminently suitable for tube drawing, in particular includingfor the drawing of tubes having the small diameters mentioned above.Therefore, the glasses for fluorescent lamps are also exceedingly wellsuited to the production of miniaturized fluorescent lamps, for examplefor the background illumination of displays, e.g. of personal computers,laptops, notebooks, palmtops, vehicle navigation systems, scanners, butalso of mirrors and pictures.

The fluorescent lamps produced using the glasses according to theinvention, in particular miniaturized fluorescent lamps, satisfy thedemands imposed on such lamps.

1. A borosilicate glass, having a coefficient of thermal expansionα_(20/300) of between 3.4×10⁻⁶/K and 4.86×10⁻⁶/K and a composition, inpercent by weight based on oxide content of: SiO₂ 55-80 B₂O₃  8-25 Al₂O₃0.5-10  Li₂O + Na₂O + K₂O  1-16 MgO + CaO + SrO + BaO 0-6 ZnO 0.6-3  Bi₂O₃   0-2.0 MoO₃   0-1.20 with Bi₂O₃ + MoO₃ 0.01-2.0; 

and which is free of ZrO₂.
 2. The borosilicate glass as defined in claim1, containing from 55-79 percent by weight of said SiO₂ and from 10-25percent by weight of said B₂O₃.
 3. The borosilicate glass as defined inclaim 1, containing from 73-79 percent by weight of said SiO₂, from12.5-25 percent by weight of said B₂O₃, and from 1-11 percent by weightof a total amount of said Li₂O+Na₂O+K₂O.
 4. The borosilicate glass asdefined in claim 1, containing from 55-73 percent by weight of saidSiO₂, from 15-25 percent by weight of said B₂O₃, from 1-10 percent byweight of said Al₂O₃, and from 4-16 percent by weight of a total amountof said Li₂O+Na₂O+K₂O.
 5. The borosilicate glass as defined in claim 1,containing from 55-79 percent by weight of said SiO₂ and from 8-12.5percent by weight of said B₂O₃.
 6. The borosilicate glass as defined inclaim 1, containing a total amount of said Bi₂O₃+MoO₃ of from 0.1 to 2.0percent by weight.
 7. The borosilicate glass as defined in claim 6 ,wherein said total amount of said Bi₂O₃+MoO₃ is from 0.2 to 2.0 percentby weight.
 8. The borosilicate glass as defined in claim 1, additionallycontaining, in percent by weight based on oxide content: Fe₂O₃ 0-1 CeO₂0-1 TiO₂ 0-5 PbO 0-3 As₂O₃ 0-1 Sb₂O₃ 0-1 with Fe₂O₃ + CeO₂ + TiO₂ +PbO + 0-5 As₂O₃ + Sb₂O₃ + V₂O₅ + Nb₂O₅ + WO₃ SnO₂ 0-1 F 0-1 Cl   0-0.5SO₃    0- 0.5.


9. The borosilicate glass as defined in claim 1, having a transformationtemperature Tg of between 440° C. and 580° C. and having a transmissionτ at λ≦240 nm of ≦0.1% for a specimen thickness of 0.2 mm.
 10. Theborosilicate glass as defined in claim 1, having a transformationtemperature Tg of between 440° C. and 580° C. and having a transmissionτ at λ≦254 nm of ≦0.1% for a specimen thickness of 0.2 mm.
 11. Afluorescent lamp produced from the borosilicate glass as claimed inclaim
 10. 12. A miniaturized fluorescent lamp produced from theborosilicate glass as claimed in claim 10.